1. Field of the Invention
The present invention relates to improvements in transfer printing, particularly to improvements in thermal transfer printing, which increase print energy efficiency and print quality. In a first embodiment an energy absorber is arranged in a transfer printing medium such that it absorbs applied energy necessary for the transfer of indicia-forming material without degrading quality and/or color characteristics of the printed indicia-forming material. In a second embodiment, the efficiency, speed and range of printing energy suppliers available for transfer printing is increased by heating a transfer printing medium prior to or during the application of print energy.
2. Discussion of the Background
Transfer printing is a generic term for processes in which one or more print indicia materials (i.e., dyes, dispersions, colorants, etc.) are caused to transfer from one medium to a receiver surface in response to an external stimulus. See U.S. Pat. Nos. 4,614,521, 4,657,557, 4,748,151, 4,771,035, 4,816,435, 4,820,685, 4,829,049, 4,895,830, 4,954,478, 4,981,748, 4,985,396, 4,988,666, 5,013,606, 5,139,995, 5,143,782, 5,189,008, 5,236,768, 5,256,492, 5,292,583, 5,308,681, 5,314,862, 5,324,583, 5,350,730, 5,004,659, and 5,008,152 (referred to hereinafter in connection with the present invention) as well as xe2x80x9cPC Computingxe2x80x9d, September 1993, especially pages 174-175 thereof, all incorporated herein by reference. Perhaps the most popular area of transfer printing is thermal transfer printing wherein thermally transferable dyes are caused to transfer from a medium such as a dyesheet to, e.g., a piece of paper. Such thermal transfer printing has been used in the past for printing woven and knitted textiles and various other rough materials by placing over the material to be printed on a sheet carrying the desired pattern in the form of sublimable dyes. Common commercial forms of such materials are xe2x80x9ciron-onxe2x80x9d images popular for children""s clothes.
A more recent thermal transfer printing process is one used to produce indicia on a surface, typically a piece of coated or plain paper, using pixel printing equipment including thermal energy sources such as thermal pin printers, or light energy sources including laser beams, etc whose energy is converted to thermal energy when absorbed. Typically, the print energy source (thermal print head, laser printer, etc.) is controlled by electronic signals derived from video, computer, electronic camera, or similar signal generating apparatus. The pattern to be printed need not be pre-formed on a print medium such as a dyesheet, and a medium is used which typically comprises a thin substrate supporting at least one transfer coat having single or multiple print indicia materials (dyes, dispersions, etc.) contained therein and forming, typically, a continuous and uniform layer over an entire printing area of the medium. Examples of printing techniques using such media include thermal wax printing, sublimation dye printing, dye diffusion thermal transfer printing, dye transfer printing, etc.
Typical examples of print media include printing ribbons such as those commonly used in office products to carry coloring materials onto a receiver sheet. Printing is typically effected by heating selected discrete areas of the medium, e.g., dyesheet, while the transfer coat is held against a dye,receptive surface such as coated or plain paper, causing a colorant, e.g., a dye, to transfer onto the corresponding areas of the receptive surface. The shape of the pattern transferred is determined by the number and location of areas which were subjected to heating, and the. depth of the shade in any area is determined by the period of time for which it is heated, and the temperature reached. Printing which is thought to rely on the diffusion of a dye to a receptive surface is typically referred to as dye diffusion thermal transfer printing, or D2T2 printing. Printing according to D2T2 is quite popular in the Uunited States, including the printing of colored indicia by this method. D2T2 printing is currently limited, however, to thermal print head, etc. print energy suppliers, the expense and power of a laser necessary to affect D2T2 printing currently being to high. Sublimation dye printing, where a dye is thought to be converted to the gas phase from the solid phase and deposited on a receiver sheet or surface, and thermal wax printing, where a wax binder is melted and transferred, are also popular. All. these printing methods are referred to herein xe2x80x9cthermal printingxe2x80x9d since printing requires thermal energy. The term xe2x80x9cprint energyxe2x80x9d as used herein includes all energies that effect thermal printing, including heat, light, etc.
Typical dyesheets useful in thermal printing, including printing ribbons, etc., generally are made from a sheet-like substrate such as paper, polymeric resins, etc. which support on one surface thereof a transfer coat containing a thermally transferable dye, typically contained within a polymeric or wax binder. Additional coating layers may also be present, including adhesive or dye-barrier sublayers between the substrate and transfer coat, and back coats on a second surface for improving slip or heat resistance properties. See, for example, the above list of U.S. patents incorporated herein by reference for examples of substrates, binders, dyes, etc. Such dyesheets may be elongated and/or housed in a cassette for convenience, enabling them to be wound so as to expose fresh areas of transfer coat after each print event has occurred.
Thermal transfer printing media designed for producing multicolored prints have a plurality of panels of different uniform colors, typically three or four, including yellow, magenta and cyan and sometimes black. When supported on a substrate elongated in the form of a ribbon these different panels may be provided as longitudinal parallel strips or as transverse panels, each the size of the desired print arranged in a repeated sequence of the color used. During printing, panels of each color are placed on or near the receiver sheet and energy is, applied thereto by means of a thermal head, laser, etc., to effect transfer of the dye as required. The initial print indicia can be overprinted by a subsequent color to make up the full color spectrum.
In order to increase the efficiency of thermal transfer printing, it is typical to include an energy absorber, typically an IR energy absorber, within the transfer coat and mixed in contact with the dye to be transferred in order to improve transfer efficiency by increasing the efficiency of energy absorption by the print-transferring medium. Unfortunately, such energy absorbers, which are typically chemical species, interfere with and change the color of the printed indicia by, e.g., interfering with the dye color. This contamination often makes it impossible to print true yellow, true magenta, true cyan, etc. and significantly impacts the application of transfer printing such as D2T2 printing. The alternative, of course, is to use no energy absorber, but this solution leads to a situation where the amount of energy applied to the print ribbon, transfer coat, etc., must be unacceptably increased, resulting in blurred images, damaged printing ribbons, etc. Moreover, and as mentioned above, due to the relatively high energy requirements necessary to effect D2T2 printing, especially with commercially available D2T2 print ribbons, the use of lasers as print energy suppliers for transfer indicia writers is generally commercially unknown.
Accordingly, one object of this invention is to provide a novel transfer printing medium which includes an energy absorber which does not interfere with the printed characteristics, especially the color, of the dye, dispersion, etc. (hereinafter referred to as colorant) transferred therefrom during printing.
Another object of the present invention is to provide a transfer printing method in which the amount of energy required to be supplied from the print head to effect transfer printing is decreased.
Another object of the present invention is to provide a transfer printing method wherein a laser beam is used as a print head to supply the print energy necessary to effect transfer of colorant from a transfer printing medium to a receiver surface.